EMC in Electrical Power Systems
Frithiof Jensen
Power System Engineer
November 12, 2013
• The EU EMC Directive 89/336/EEG mandates control of
EMC in electrical equipment
• The EMC Directive specifies two essential requirements:
– The device must not interfere with radio or telecommunications
equipment operation and must not perturb the environment
– The device itself must be immune from electromagnetic
disturbance due to itself or other equipment (Immunity).
• From 20 july 2007, the EMC Directive also applies to
equipment that is not intended to be sold in the market
and is not CE-marked, f.ex. fixed installations and
specialised industrial plants and equipment as used
within the ESS.
EMC-directive (cntd.)
• Supply voltages
– Electrical supply limits are given in SS-EN 50150 ”Voltage
characteristics of electrity supplied by public electricity networks”
• EMC – limits for emission and susceptibility
– SS-EN 61000-2-2 – specifies tolerance levels for conducted
disturbances in low voltage distribution systems
– SS-EN 61000-2-4 – defines electrical environment and tolerance levels
for industrial- and private- distribution systems
– SS-EN 61000-2-12 - defines electrical environment and tolerance levels
medium voltage distribution systems
– Many, many & Many product-specific standards in the 61000- family!
Electrical system
SS-EN 610002-2
SS-EN 50150
• What is it?
– Flicker is the name for rapid, repetitive voltage changes in electrical
– The severity depends on magnitude and frequency; at the 14 Hz operating
frequency of the LINAC, the acceptable voltage change is 0.3%
• What causes flicker?
– Flicker is caused by load variations; for example arc welding, electrical
smelters and klystron modulators
• Effect of flicker
– Mainly visual, the lights are “blinking”. Severe Flicker can confuse the
power factor correction logic in switch mode power supplies
• Prevention of flicker
– Increase the short-circuit power of the network
– Reduce load power variations so that the flicker level is below the
• What is it?
– Overtones are frequencies higher than the 50 Hz power line frequency
superimposed onto the phase voltage or current
• What causes overtones?
– Overtones are caused by non-linear loads, imbalanced three-phase loads (due to
saturation) and power electronic equipment
• Effect of overtones
Increased power losses in cables, motors and distribution transformers
Third harmonics add up in the Neutral conductor and cause overheating
Harmonics may cause migrating currents due to impedance issues
False triggering of phase controllers and power factor correction logic
Can cause compensation capacitors to overheat
• Prevention of overtones
– Use Power Factor Correction and EMC filtering on power electronics (this is the
already the standard for CE-marked- and also most industrial equipment)
– Use 12- or 24- pulse rectifiers for large loads
– Increase the area of Neutral and Earth connectors
Voltage imbalance
• What is it?
– The phase voltages on the same 3-phase system are different
• What causes voltage imbalance?
– Voltage imbalance is caused by different impedances in the electrical
distribution system and/or unbalanced loads
• Effect of voltage imbalance
– Torque variation in 3-phase motors and high currents in the neutral
conductor – the phases no longer add up to zero.
– Overloading of branches in 3-phase power electronics
– Disturbance of SCR-triggering in rectifiers
– Saturation of distribution transformers, generating harmonics and
• Prevention of voltage imbalances
– Distribute loads evenly between phases
• What is it?
– Transients are rapid voltage changes on the phase voltages
• What causes transients?
– Lightning or rapid load variations can cause transients, especially
inductive loads
• Effect of transients
– Transients can damage equipment by exceeding the voltage
rating of components and insulation
• Prevention of transients
– Control the operation of equipment, f.ex. capacitor banks and
large motors
– Use transient voltage limiters at sensitive equipment and at the
input port of the distribution system.
Voltage Dips
• What is it?
– A random drop in phase voltage below 10% of nominal value for a duration
between 10 ms – 90 seconds. One or more phases may be affected.
• What causes dips?
– Dips are generated by lightening protection activation and switching operations in
the electrical network
• Effect of voltage dips
– Dips up to 150 ms can usually be absorbed by small PFC-type power supplies
– Larger ( > 5 kW) power supplies will normally trip because of load imbalance e.t.c.
• Prevention of voltage dips
– Voltage dips cannot be prevented. Instead equipment must be designed to tolerate
them, for example:
– Specify/Confirm adequate hold-up time for small power supplies
– Specify adequate hold-up time for large equipment
– Balance the cost of immunity with the frequency and severity of the disturbances
– NOTE: The electrical supplier is not required to measure or care about dips shorter
than 150 ms
What to expect?
• Nominal voltage
400, 690 or 6600 V
Maximum voltage variations
+/- 10 %
Typical voltage variations
+/- 5 %
Backup power
+ 10 % / -15 %
Random step changes of 1.5% (increase and decrease) of the supply
voltage may occur due to operation of the tap-changers of the main
• Nominal frequency
– Maximum variations
– Backup power
50 Hz, +/- 1 %
50 Hz, +4 % / −6 %
50 Hz ± 15 %
• Total Harmonic Distortion, THD
– Maximum value
– Typical value
– Voltage unbalance
8 %, 40’th overtone inclusive

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